The
compressive strength and the modulus of elasticity were determined from 6 x 12
in. cylinders. The compressive load of
the cylinder with silica fume was 6 061 psi while the compressive load of the
cylinder without silica fume was 5 628 psi. The compressive stress for the silica
fume beam was 6 061 psi while the stress for the non-silica fume beam was 5 628
psi. Silica fume is a pozzolan, which
in the presence of water, chemically reacts with calcium hydroxide released by
the hydration of Portland cement to form compounds possessing cementitious
properties. So the excepted load of
concrete with silica fume should be higher than concrete without silica
fume. The compressive strength falls in
the higher range for general use concrete, which normally has a compressive
strength between 3 000 psi and 5 000 psi; this is just below the compressive
strength for high strength concrete, which starts at 6 000 psi. The cylinders
failed in a cone with approximately 45 degree sides, as was expected from the
Mohr’s stress circle for the cylinders. The experimental value of the modulus
of elasticity, E, was determined to be 2.61*106 psi. While E based
on the ACI equation with silica fume was 25.32*106 psi, E from the
ACI equation without silica fume was 23.91*106 psi. The experimental
value of the E does not match the ACI equation because the ACI equation is
based on a 28-day moist curing period, while the experiment only had a 14-day
moist curing period.
Two cylinders
were capped with sulfate so that the bottom is perpendicular to the sides so
that when the load is applied, it will be uniaxially applied.
The Schmidt-Hammer test measures the
surface hardness of concrete, which allows a compressive strength to be
estimated. Density is a good indication
of strength. As the water/cement ratio
goes up porosity goes up and strength goes down. This is related to compressive, tensile, bending strength and
shear. As slump and amount of water go
up, strength goes down. As air content
goes up, unit weight and strength go down.
Mix design
leads to different mechanical properties just as testing conditions affect the
apparent strength. Load being applied
at a fast rate gives a higher apparent strength, while loading at a slow rate
gives a lower apparent strength.
Concrete fails by micro cracking, therefore the longer it takes to put
the load on the more time there is for the fault to break. When the load is applied quickly there is
less time for the faults to develop.
In the second
test performed, the split cylinder test, the cylinder’s split lengthwise to
approximate a value for tensile strength.
The maximum load for the specimen with silica fume was 39 000 lbs. The maximum load for the non-silica fume
specimen was 38 000 lbs. The modulus of
elasticity was found to be 23.91x10^6.
The ultimate strength for the silica fume was 344.8 psi, whereas the
non-silica fume cylinder had an ultimate strength of 336 psi; therefore the
values fall within the specified ACI guidelines.
The third
test performed was used to measure the flexural strength of a concrete
beam. The 6 x 6 in specimen was loaded
in a three-point bend test with a Modulus of Rupture of 1411.76 psi and an
ultimate strength of 289.7 psi. The
first 3 x 3 in. beam broke while it was being loaded into the testing
machine. The second 3 x 3 in. beam,
subjected to a four-point bend test, had a Modulus of Rupture of 735.4 psi and
an ultimate strength of 121 psi. The
third 3 x 3 in. beam, also subjected to a four-point bend test, resulted in a
Modulus of Rupture of 917.6 psi and a strength of 133.2 psi.